Our solar system
is composed of the Sun and all things which orbit around it: the
Earth, the other eight planets, asteroids, and comets. The Sun is 150
million kilometers (93 million miles) away from the Earth (this
distance varies slightly throughout the year, because the Earth's
orbit is an ellipse and not a perfect circle).

The Sun is an average star - there are other stars which are much hotter or much cooler, and
intrinsically much brighter or fainter. However, since it is by far
the closest star to the Earth, it looks bigger and brighter in our sky
than any other star. With a diameter of about 1.4 million kilometers
(860,000 miles) it would take 110 Earths strung together to be as long
as the diameter of the Sun.
The Sun is mostly made up of hydrogen (about 92.1% of the
number
of atoms, 75% of the mass). Helium can also be found in the Sun (7.8%
of the number of atoms and 25% of the mass). The other 0.1% is made
up of heavier elements, mainly carbon, nitrogen, oxygen, neon,
magnesium, silicon and iron. The Sun is neither a solid nor a gas but
is actually plasma. This plasma is
tenuous and gaseous near the surface, but gets denser down towards the
Sun's fusion core.

The Sun, as shown by the illustration to the left, can be divided into six layers. From
the center out, the layers of the Sun are as follows: the solar
interior composed of the core
(which occupies the innermost quarter or so of the Sun's radius), the
radiative zone, and the
the convective zone,
then there is the visible surface
known as the photosphere, the
chromosphere, and finally the outermost layer,
the corona.
The energy produced through fusion in the Sun's core powers the Sun and produces all of the heat
and light that we receive here on Earth. The process by which energy
escapes from the Sun is very complex. Since we can't see inside the Sun,
most of what astronomers know about this subject comes from combining
theoretical models of the Sun's interior with observational facts such
as the Sun's mass, surface temperature, and luminosity (total amount
of energy output from the surface).

All of the energy that we detect as light and heat originates from nuclear reactions deep
inside the Sun's high-temperature "core." This core extends about one quarter of the way
from the center of Sun (where the temperature is around 15.7 million kelvin (K), or 28 million
degrees Fahrenheit) to
its surface, which is only 5778 K "cool".

Above this core, we can
think of the Sun's interior as being like two nested spherical shells
that surround the core. In the innermost shell, right above the core,
energy is carried outwards by radiation. This "radiative zone"
extends about three quarters of the way to the surface.
The radiation
does not travel directly outwards - in this part of the Sun's
interior, the plasma density is very high, and the radiation gets
bounced around countless numbers of times, following a zig-zag path
outward.

It takes several hundred thousand years for radiation to make its
way from the core to the top of the radiative zone!
In the outermost of the two shells, where the temperature
drops
below 2,000,000 K (3.5 million degrees F) the plasma in the Sun's
interior is too cool and opaque to allow radiation to pass. Instead,
huge convection currents form
and large bubbles of hot plasma move up towards the surface (similar
to a boiling pot of water that is heated at the bottom by a stove).
Compared to the amount of time it takes to get through the radiative
zone, energy is transported very quickly through the outer convective
zone.

The Sun's visible surface the photosphere is "only" about
5,800
K (10,000 degrees F). Just above the photosphere is a thin layer
called the chromosphere. The name chromosphere is derived from the
word chromos, the Greek word for color. It can be detected in
red hydrogen-alpha light meaning that it appears bright red. Above
the surface is a region of hot plasma called the corona. The corona
is about 2 million K (3.6 million degrees F), much hotter than the
visible surface, and it is even hotter in a flare. Why the atmosphere
gets so hot has been a mystery for decades; SOHO's observations are
helping to solve this mystery.

The Sun is not just a big bright ball. It has a complicated
and changing magnetic field,
which forms things like sunspots and active
regions. The magnetic field sometimes changes explosively, spitting
out clouds of plasma and energetic
particles into space and sometimes even towards Earth.
The solar magnetic field changes on an 11 year cycle. Every
solar
cycle, the number of sunspots, flares, and solar
storms increases to a peak, which is known as the solar maximum. Then, after a
few years of high activity, the Sun will ramp down to a few years of
low activity, known as the solar
minimum. This pattern is called the "sunspot
cycle", the "solar cycle", or the "activity cycle".
Stars like the Sun shine for nine to ten billion
years. The Sun is about 4.5 billion years old, judging by the age of
moon rocks. Based on this information, current astrophysical theory
predicts that the Sun will become a red giant in about five billion
(5,000,000,000) years.